The subject matter of this invention relates to the field of energy storage and utilization with novel compositions of matter that reversibly store hydrogen as a source of electrochemical energy for subsequent release to produce an electrical current. More particularly, novel active material compositions, processes of making the active material, fabrication and assembly of electrodes, cells, and batteries are disclosed herein.
Some research has been conducted involving hydrogen storage secondary batteries. However, a basic understanding resulting in a viable approach to optimizing such batteries hs not been forthcoming in the scientific or patent literature. Examples of such efforts are U.S. Pat. Nos. 3,669,745 and 3,824,131 and a technical paper entitled "A New Type of Reversible Negative Electrode for Alkaline Storage Batteries Based on Metal Alloy Hydrides," 1974, 8th International Power Sources Conference. These research efforts have not resulted in widespread commercial utilization of this battery technology. In fact, the prior research suggests no significant improvement over conventional battery systems such as nickel cadmium. As a result, the hydrogen storage battery system has apparently been ignored or abandoned.
Secondary batteries using a hydrogen rechargeable electrode operate in a different manner than lead acid, nickel cadmium, or other battery systems. The hydrogen storage battery utilizes an anode which is capable of reversibly electrochemically storing hydrogen and usually employs a cathode of nickel hydroxide material. The anode and cathode are spaced apart in an alkaline electrolyte. Upon application of an electrical current to the anode, the anode material (M) is charged by the absorption of hydrogen: EQU M+H.sub.2 O+e.sup.- M--H+OH.sup.-
Upon discharge the stored hydrogen is released to provide an electric current: EQU M--H+OH.sup.- M+H.sub.2 O+e.sup.-
The reactions are reversible and this is also true of the reactions which take place at the cathode. As an example, the reactions at a conventional nickel hydroxide cathode as utilized in a hydrogen rechargeable secondary battery are as follows: EQU Charging: Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.- EQU Discharging: NiOOH+H.sub.2 O+e.sup.- Ni(OH).sub.2 +OH.sup.-
The battery utilizing an electrochemically hydrogen rechargeable anode offers important potential advantages over conventional secondary batteries. Hydrogen rechargeable anodes should offer significantly higher specific charge capacities than lead anodes or cadmium anodes. Furthermore, lead acid batteries and nickel-cadmium type secondary batteries are relatively inefficient, because of their low storage capacity and cycle life. A higher energy density should be possible with hydrogen storage batteries than these conventional systems, making them particularly suitable for battery powered vehicles and other mobile applications. Hydrogen storage batteries have not lived up to their potential, however, because of the materials and mechanical structures used.
An example of hydrogen storage materials which are not readily useable for battery applications is found in Japanese Patent Publication No. Sho53-164130 which was published July 11, 1980. A hydrogen storage metal material is disclosed with the composition formula EQU (V.sub.1-x Ti.sub.x).sub.3 Ni.sub.1-y M.sub.y,
whereas M is Cr, Mn, Fe; 0.05.ltoreq..times..ltoreq.0.8 and 0.ltoreq.y.ltoreq.0.6. The temperature and pressure conditions for using this material for effective hydrogen storage, however, exceed the normal conditions at which commercially acceptable batteries safely operate. Other problems, like corrosion also must be alleviated if these hydrogen storage materials are used in a battery.
The preparation of hydrogen storage materials and fabrication of electrodes also are of utmost importance. It is desirable that the hydrogen storage materials be somewhat homogeneous to provide uniformity in their electrochemical properties. Often the individual components of the hydrogen storage materials are combined by melting the components together to form a bulk material such as an ingot. The hydrogen storage materials produced in this form are unsuitable for immediate use without further processing. Reducing the size of these bulk materials for fabrication as an electrode, however, can be quite difficult because of the unusual hardness and ductility of many hydrogen storage materials. Normal size reduction techniques which use such devices as jaw crushers, mechanical attritors, ball mills, and fluid energy mills often fail to economically reduce the size of such hydrogen storage materials. Thus, grinding and crushing operations for these materials have been complicated and the results have not been uniform.
Attempts to make metals brittle in order to crush them more easily are not new in the art. Prior methods, however, have involved mechanical addition of embrittling agents, the presence of which would have an undesirable effect on the electrochemical properties of the hydrogen storage materials.
Other methods for embrittling metals are disclosed in Canadian Pat. No. 533,208 granted to Brown. This patent identifies many disadvantages of treating vanadium metal with hydrogen gas to facilitate its crushing and, instead, recommends using cathodic charging as a successful size reduction technique. Although one is dissuaded from using hydrogen gas by the Brown patent, the present invention overcomes the disadvantages to provide a useful and commercially desirable technique of size reduction.
The previous attempts to utilize hydrogen storage materials in secondary batteries have proven unsuccessful because of the materials' poor electrochemical performance, structural instability, and expensive fabrication. The invention herein provides a new and improved battery and method of fabricating the same with an electrode incorporating an active material composition and structure allowing for high charge and discharge rates, efficient reversibility, high electrical efficiency, bulk hydrogen storage without substantial structural change or poisoning, mechanical integrity over long cycle life, and deep discharge capability.